156 ZOOLOGY. 



layer is thrown off (this probably corresponding to the serous 

 membrane of insects and Crustacea) ; the central mass 

 (which is not hollow as in the gastrula of other worms, a 

 digestive cavity not being present in after life) forms the 

 embryo, and soon three pairs of hooks arise. Three struc- 

 tureless membranes are secreted around the embryo, which 

 then hatches. The embryo of Bothriocephalus is provided 

 with a ciliated membrane, which corresponds to the first 

 blastodermic moult of the embryo Tsenia, which, on the 

 other hand, is not ciliated. 



The history of the human tape- worm, Tmnia solium (Fig. 

 103) is as follows : the eggs eaten by the hog are developed 

 in its body into the larval tapeworm (scolex), called in this 

 species Oysticercus cellulosce (Fig. 105 ; Fig. 106, head en- 

 larged). The head with its suckers is formed, and the 

 body becomes flask-shaped ; the Cysticerci then bury them- 

 selves in the liver or the flesh of, pork, and are transferred 

 living in uncooked pork to the alimentary canal of man. 

 The body now elongates and new joints arise behind the head 

 until the form of the tapeworm is attained, as in Fig. 103. 



The hinder joints then become filled with eggs and break 

 off, becoming independent zooids comparable with the 

 " parent-nurses " of the Cercarias, except that they are not 

 contained in the body of the Tffinia (as in the Cercaria), but 

 are set free. The independent joint (Figs. 104, 107) is 

 called a " proglottis. " It escapes from the alimentary tract 

 of its human host, and the eggs set free, in and about 

 privies, are swallowed by that unclean animal, the pig, and 

 the cycle of generations begins anew. "We thus have the 

 following series of changes, which may be compared with 

 the homologous series in the flukes : 



1. Egg. 



2. Morula. 



3. Double- walled sac (gastrula ?). 



4. Proscolex, free embryo with hooks, surrounded by a 

 blastodermic skin. 



5. Scolex (Oysticercus, larva). Body few-jointed. 



6. Strobila (Tsenia). Body many-jointed. 



7. Proglottis (adult). 



